Rapid Strengthening of Interstitial Free Steel Using Amorphous FeC Thin Films and Induction Heating

A new process to rapidly obtain high-strength interstitial free (IF) steel was investigated. Thin sheets of IF steel were coated on one or both sides with an amorphous FeC film and subjected to a two-step induction heating cycle (1100 °C followed by an isothermal hold at 780 °C for 2 or 4 min) and a rapid quench in water. Tensile mechanical properties were measured, and a yield stress of 374 MPa and an ultimate tensile strength of 448 MPa were achieved after 2 min of induction heating. After 4 min of induction heating, the yield stress and the ultimate tensile strength drop at 206 and 320 MPa, respectively. During tensile testing, the specimens induction heated for 2 min show Lüdering, which is suppressed when the induction heating is extended to 4 min. Vickers microhardness measurements through thickness confirm that higher mechanical properties are obtained after 2 min of induction heating. Transmission electron microscopy reveals that strengthening results from dislocations, carbon in solid solution, and the precipitation of nanosized TiC particles. A fine microstructure with an average grain size of 15 μm is preserved after the induction heat treatment.